A major source of methane is natural gas which typically contains about 40-95% methane depending on the particular source. Other constituents include about 10% ethane with the balance being made up of CO.sub.2 and smaller amounts of propane, butanes, pentanes, nitrogen, etc.
Primary sources for natural gas are reservoirs either alone or generally associated with hydrocarbon liquid reserves. Most of the natural gas used for heating purposes comes from these sources. Quantities of natural gas are also known to be present in coal deposits and are by-products of crude oil refinery processes and bacterial decomposition of organic matter.
Prior to commercial use, natural gas must be processed to remove water vapor, condensible hydrocarbons and inert or poisonous constituents. Condensible hydrocarbons are generally removed by cooling natural gas to a low temperature, and then washing it with a cold hydrocarbon liquid to absorb the condensible hydrocarbons. The condensible hydrocarbons are typically ethane and heavier hydrocarbons. This gas processing can occur at the wellhead or at a central processing station. Processed natural gas typically comprises a major amount of methane, and minor amounts of ethane, propane, butanes, pentanes, carbon dioxide and nitrogen. Generally, processed natural gas comprises from about 70% to more than about 95% by volume of methane.
Most processed natural gas used commercially is distributed through extensive pipeline distribution networks. As natural gas reserves in close proximity to gas usage decline, new sources that are more remote require transportation over further distances. The gas from many of these distant sources is not, however, amenable to transport by pipeline. For example, the gas from sources that are located in areas requiring economically unfeasible pipeline networks, or in areas requiring transport across large bodies of water, is not amenable to transport by pipeline. This problem has been addressed in several ways. One such solution has been to build a production facility at the site of the natural gas deposit to manufacture one specific product. Another approach has been to liquefy the natural gas and transport it in specially designed tanker ships. Natural gas can be reduced to 1/600th of the volume occupied in the gaseous state by cryogenic processing, and with proper procedures, safely stored or transported. These processes, which involve liquefying natural gas, transporting the liquified gas, and revaporizing it are complex, energy intensive and expensive.
Still another approach has been the conversion of natural gas to higher molecular weight hydrocarbons or oxygenates, preferably substantially liquid hydrocarbons or oxygenates, that can be easily handled and transported. The conversion of natural gas to higher order hydrocarbons, especially ethane and ethylene, retains the material's versatility for uses as precursor materials in chemical processing. Known dehydrogenation and polymerization processes are available for the further conversion of ethane and ethylene to liquid hydrocarbons. In these ways, easily transportable commodities may be derived from natural gas at the wellhead. A drawback in implementing such processes has been the lack of means for obtaining a sufficiently economical conversion rate of natural gas to higher molecular weight hydrocarbons.
The conversion of methane to higher molecular weight hydrocarbons at higher temperatures, in excess of about 1200.degree. C., is known. These processes are, however, energy intensive and have not been developed to the point where high yields are obtained even with the use of catalysts. Some catalysts or promoters that are useful in these processes, e.g. chlorine, are corrosive under such operating conditions.
Low temperature reactions, e.g. 250.degree. C. and 500.degree. C., of hydrocarbon feedstocks to higher molecular weight hydrocarbons are described in U.S. Pat. Nos. 4,433,192; 4,497,970 and 4,513,164. The processes described in these patents utilize heterogeneous systems and solid acid catalysts. In addition to the solid acid catalysts, the reaction mixtures described in the -970 and -164 patents include oxidizing agents. Among the oxidizing agents disclosed are air, O.sub.2 -O.sub.3 mixtures, S, Se, SO.sub.3, N.sub.2 O, NO, NO.sub.3, F, etc.
One area of active interest has been labelled oxidative coupling, and generically consists of promoting the following reaction: EQU Natural Gas+Oxygen.fwdarw.Hydrocarbons+Byproducts
where Natural Gas is used to represent natural gas or its components, Oxygen is used to represent either molecular or chemically bound oxygen, Hydrocarbons is used to represent species containing more than one carbon atom, and Byproducts represents water, carbon oxides and solid carbonaceous materials. Proposed configurations for accomplishing the above reaction vary, including cofeed, where oxygen and methane are mixed and reacted either directly or in the presence of a catalyst, and redox, where a solid ferries oxygen from one vessel (generator) to another (reactor) in which the desired oxidative coupling takes place. Many examples of these configurations exist in the patent and technical literature, including numerous patents by Exxon, Atlantic Richfield, Chevron, ARCO Chemical Company and others.
See for example, U.S. Pat. Nos. 4,754,093; 4,751,336; 4,704,496; 4,613,426; 4,599,478; 4,599,479; 4,587,001; 4,556,749; 4,527,002; 4,527,003; 4,520,224; 4,430,096; 4,288,408; and 3,900,525.
The catalytic oxidative coupling of methane at atmospheric pressure and temperatures of from about 500.degree. C. to 1000.degree. C. has been investigated by G.E. Keller and M.M. Bhasin. These researchers reported the synthesis of ethylene via oxidative coupling of methane over a wide variety of metal oxides supported on an alpha-alumina structure in Journal of Catalysis, 73, 9-19 (1982). This article discloses the use of single component oxide catalysts that exhibited methane conversion to higher order hydrocarbons at rates no greater than 4%. The process by which Keller and Bhasin oxidized methane was cyclic, alternating the feed composition between methane and nitrogen and air (oxygen) to obtain higher selectivities.
The conversion of methane to higher molecular weight hydrocarbons using metal oxide catalysts and oxides of carbon, which are generated from the hydrocarbon, is also described in U.S. Pat. No. 2,180,672. The conversion generally is carried out at temperatures of from about 150.degree.-350.degree. C., and the oxides of carbon are consumed in the reaction.
U.S. Pat. No. 1,677,363 describes the conversion of methane or natural gas to ethylenic hydrocarbons by heating a thin stream of methane or natural gas to a temperature not exceeding 950.degree. C. U.S. Pat. No. 4,304,657 describes a process for converting feedstocks comprising aliphatic fractions boiling at 70.degree. C. Typically, the feedstock may be naphthas, coker gasolines, FCC gasoline, and pyrolysis gasolines. The process uses aromatization catalysts and a diluent which may be CO.sub.2, CO or nitrogen, and the dilution is in a molar ratio of diluent to feed of from about 20:1 to 1:1. Preferred dilutions are 10:1 to 5:1 of diluent to feed.
Methods for converting methane to higher molecular weight hydrocarbons at temperatures in the range of about 500.degree. C. to about 1000.degree. C. are disclosed in U.S. Pat. Nos. 4,443,644; 4,443,645; 4,443,646; 4,443,647; 4,443,648 and 4,443,649. The processes taught by these references provide relatively high selectivities to higher order hydrocarbons, but at relatively low conversion rates, on the order of about less than 4% overall conversion. In addition to synthesizing hydrocarbons, the processes disclosed in these references also provide a reduced metal oxide which must be frequently regenerated by contact with oxygen. The preferred processes of these references entail physically separate zones for a methane contacting step and for an oxygen contacting step, with the reaction promoter recirculating between the two zones.
U.S. Pat. Nos. 4,172,810; 4,205,194; and 4,239,658 disclose the production of hydrocarbons including ethylene, benzene, ethane, propane, and the like in the presence of a catalyst-reagent composition which comprises: (1) a group VIII noble metal having an atomic number of 45 or greater, nickel or a group Ib noble metal having an atomic number of 47 or greater; (2) a group VIb metal oxide which is capable of being reduced to a lower oxide; and (3) a group IIa metal selected from the group consisting of magnesium and strontium composited with a passivated, spinel-coated refractory support or calcium composited with a passivated, non-zinc containing spinel-coated refractory support. The feedstreams used in the processes disclosed in these patents do not include gaseous oxygen. Oxygen is excluded for the purpose of avoiding the formation of carbon oxides from useful intermediate hydrocarbon compounds. Oxygen is generated for the reaction from the catalyst, thus requiring periodic regenerations of the catalyst.
U.S. Pat. No. 4,450,310 discloses a methane conversion process for the production of olefins and hydrogen comprising contacting methane in the absence of oxygen and in the absence of water at a reaction temperature of at least 500.degree. C. with a catalyst comprising the mixed oxides of a first metal selected from the group consisting of lithium, sodium, potassium, rubidium, cesium and mixtures thereof; a second metal selected from the group consisting of beryllium, magnesium, calcium, strontium, barium, and mixtures thereof; and optionally, a promoter metal selected from the group consisting of copper, rhenium, tungsten, zirconium, rhodium, and mixtures thereof.
In general, the oxidative coupling processes operate at moderately high temperatures, typically 600.degree.-1000.degree. C. in catalytic systems, and temperatures in excess of 1000.degree. C. in non-catalytic systems, and characteristically have conversions per pass of 1-40 percent of the contained methane. Selectiveness to the C.sub.2 + products or intermediates range from 20-90+ percent. In general, if a catalyst is run to maximize selectivity, it will exhibit lower conversion, and running under conditions that increase conversion results in a reduction in selectivity.
The combination of moderate conversion per pass and the characteristic production of quantities of excess carbon oxides obtained in the prior art processes limits the utility of present oxidative coupling approaches. Particularly important is the tendency of both the catalytic and non-catalytic systems to produce significant quantities of carbon oxides which are of relatively low value. Increasing levels of conversion per pass usually increases the problem of carbon oxide production.
Some of the selectivity loss can be recaptured by reacting any hydrogen produced in the process with some of the co-produced carbon oxides to manufacture oxygenates or, directly or indirectly, higher hydrocarbons. However, much of the hydrogen is usually converted to water and is unavailable for the conversion of carbon oxides to useful products. In general, carbon oxide conversion can increase useful product yields by up to 3% based on previously published yield structures.
An equally important limitation in prior art processes is the necessity of running the oxidative coupling reaction at elevated temperatures, and then rapidly cooling or quenching the reactor product to reduce the occurrence of severe retrograde reactions. Typically the temperature of the reactor effluent must be lowered to 700.degree. C. or even 600.degree. C. to reduce the formation of carbonaceous deposits, the hydrogenation of produced olefins to lower value aliphatic compounds, etc. This temperature reduction generally is accomplished in from less than one second to a few seconds by quenching with water or other heat absorbing medium. This approach has major drawbacks which the present invention overcomes. Specifically, valuable high quality (high temperature) heat is wasted, and low quality (low temperature) heat, which must then be disposed of at significant cost, is increased. Further, at attractive selectivities to desired intermediate or end products, the present state of the art provides, at best, approximately 25% conversion rate. In view of the low conversion rates, coupled with the other drawbacks discussed above, there remains a need for an efficient, cost effective, improved method of converting methane to higher hydrocarbons via oxidative coupling. The present invention provides such a process.
Thermal or physical quenching has heretofore been employed in prior art cracking processes involving gross volumes of heavy hydrocarbon feeds where heat is fed into the system to promote feedstock cracking, after which the reaction is quenched to prevent retrograde reactions. Generally speaking, prior art processes that employ hydrocarbons as a heat sink are doing so as a mechanism to reduce process stream volume and the size of heat recovery equipment downstream which is required if water vaporization is used to absorb heat and lower stream temperatures. Such prior art techniques, however, including equipment are those described in U.S. Pat. Nos. 4,520,224, 4,288,408, 4,556,749 and 4,384,160, all rely on specific heat capacity, phase change, heat of vaporization, and the like to dilute the concentration of heat thus lowering stream temperature. These prior art processes produce lower quality heat downstream resulting in the requirement of more heat rejection equipment in the form of cooling towers, vaporizers, heat exchangers and the like. Further, the prior art cracking processes which inadvertently employ some form of chemical quenching, that is, use a chemical agent to achieve a thermal or physical quench, as opposed to a true chemical quench as employed in the present invention, produce methane in a form only useful as process fuel, the very material the present invention is designed to convert to more transportable, higher hydrocarbons.
In the present invention, on the other hand, high quality heat is absorbed in the upgrading of the quench material to a useful product or intermediate. In addition, hydrogen produced as a result of the conversion of the quench material can be used to convert otherwise low value or waste carbon oxides to desired products or intermediates.
Thus, the present invention overcomes drawbacks of the prior art and improves the economic feasibility of both catalytic and non-catalytic oxidative coupling processes, including both cofeed and redox configurations, by recapturing the high quality heat wasted in the prior art processes, and by providing additional hydrogen which can be used to convert low value or waste carbon oxides to more valuable products.